The present invention relates to the field of supporting or retaining natural or artificial masses of more or less movable material that, due to erosion, vibration, cracking, or cleavage, is liable to suffer from landslides that are harmful to themselves and/or to the surroundings.
Although numerous implementations can be envisaged, a preferred application of the invention lies in retaining the front faces of earthworks that are cut or filled relative to a site to be protected, in particular roadways or railway lines, without this list being exhaustive.
To solve the above problem, various technical proposals have been made based on the idea that it is appropriate to raise a screen in the form of a wall that performs the function of retaining a mass of material behind an upstream, back, or xe2x80x9cwrongxe2x80x9d face.
It is rapidly apparent that such screens need to withstand horizontal stresses coming from the retained earth.
To satisfy that requirement, one prior technical proposal is to make screens in form of gravity walls, by analogy with the design that has been developed for making water-retaining dams. In that concept, the wall""s own weight is the only element providing resistance against the thrust to be withstood.
Although such walls are capable of achieving the intended objective, they are heavy and expensive to construct and in practice they are unsuitable for use with heights that are relatively large or in sites that present difficulties for moving heavy and powerful hoisting equipment.
It has been envisaged to make the mass of material itself contribute to enabling the screen or wall to withstand thrust. Thus, proposals have been made to implement the screen in the form of a thin web that can be vertical or sloping rising from an integral or separate footing possessing an upstream portion referred to as a xe2x80x9cheelxe2x80x9d which is buried in the material that is to be retained and which thus serves to provide anchoring. In general, counterforts are provided on the back face of the web to contribute to its mechanical strength.
Such a proposal appears to be more practical and less expensive, but it still gives rise to problems when it is necessary to satisfy retention requirements over a great height.
In addition, it can be observed that such proposals give no opportunities for improving the appearance of the front face of the retaining wall, e.g. by including niches for plants.
Another prior art technique proposes building retaining screens from stackable prefabricated modules.
For relatively low heights, such proposals are generally satisfactory. However, the use of stackable prefabricated modules does not make it possible to control the forces that act on them by means of the mass of material that is to be contained.
Thus, developments of the prefabricated module technique have given rise to the addition of reinforcing elements based on the reinforced earth technique, and comprising anchoring means which are inserted in the mass of material to be contained and which are also attached to the stackable prefabricated modules in such a manner as to be capable of working in traction.
The subject matter of the invention relates to that particular technique for reinforcing retaining walls.
The techniques that have been proposed for reinforcing retaining walls constituted by stackable prefabricated modules make use of several proposals.
One of them consists in securing the back face of the retaining wall to a welded wire mesh which is incorporated in various substantially horizontal planes superposed within the material to be contained.
Such a technique is not easy to implement and gives rise to a problem of distributing forces amongst the various attachment points between the mesh and the modules. It has been found that the very use of this technique causes different attachment points to be stressed differently and as a result that are uncontrollable force distributions that lead to zones of the retaining wall being associated with portions of the mesh that are not anchored or that are poorly anchored in the material to be contained.
Proposals have also been made, in particular in international patent application WO 94/23136 to make each reinforcing element in the form of two longitudinal bars that are united by spacers and form at one end loops or the like which constitute attachment parts that can be secured to the back face of the facing by means of bars fitted in appropriate manner to all or some of the stacked prefabricated modules.
That solution is found to be unsatisfactory in many respects.
Firstly, making a reinforcing element on that principle is relatively expensive since the element has the same structure over its entire length and because it needs to be anchored over its entire length by means of the materials that are to be contained.
Given this requirement, the cost price of making it is not negligible due to the raw materials used and the cost of fabrication.
In addition, account needs to be taken of standards which put a limit on the spacing of the bars constituting such elements at a determined value which is generally about 15 centimeters (cm). When such an element is to be used for interconnecting two prefabricated modules that are placed side by side, it is necessary to provide through holes for passing the vertical bars, which holes are situated at a short distance from the transverse edges of such modules. This unavoidably gives rise to a risk of breakage under tension because of the relatively small mass presented locally by the prefabricated modules.
An object of the invention is to propose a novel method of constructing a retaining wall so as to overcome the drawbacks associated with prior methods.
Another object of the proposal of the invention is to provide a novel reinforcing element suitable for overcoming the above-mentioned drawbacks while being capable of providing an improved reinforcing function by establishing a higher anchoring force while using an element that is simple, quickly made, and low in cost.
To achieve the above objects, the invention provides a method of reinforcing a retaining wall comprising a facing made up of juxtaposed component elements in front of a mass to be retained, which mass comprises immediately behind the facing an xe2x80x9cactivexe2x80x9d zone behind which there is a xe2x80x9cstrongxe2x80x9d zone, the two zones being considered as meeting in a boundary zone defined by a pseudo-plasma potential slip surface whose distance from the back face of the facing increases going from the base to the top of the retaining wall, the method being characterized in that it consists in connecting the back face of the facing to the strong zone by means of a plurality of reinforcing elements disposed substantially horizontally in superposed planes, and in selecting said elements in such a manner that each of them comprises an anchoring portion that is included in the strong zone and that is constituted by a mesh comprising more than two longitudinal bars, of which only some extend as far as the facing to define a portion that works in traction, which portion has an attachment part for connection to the facing at its end remote from the anchoring portion.
The invention also provides a reinforcing element enabling the above method to be implemented, such an element being characterized in that the anchoring portion included in the strong zone is constituted by a mesh comprising more than two longitudinal bars, only some of which extend as far as the facing to define a portion that works in traction and that presents, remote from the anchoring portion, an attachment part for attachment to the facing.
The invention also provides a retaining wall made by the method of the invention and with at least some reinforcing elements of the invention.
Thus, the retaining wall of the type comprising horizontally and/or vertically juxtaposed prefabricated elements to form a facing secured to a mass to be retained by reinforcing elements inserted in said mass is characterized in that:
each prefabricated element comprises a plate or panel possessing a front face and a back face from which there projects at least one rib having through holes on axes orthogonal to that of the rib;
at least one transverse bar is engaged through at least one hole in a rib of at least one element so as to extend on either side of said rib; and
at least one reinforcing element of the invention is secured at least to said bar so as to extend substantially horizontally away from the back face and so as to be inserted in the mass to be retained, thereby acting as a portion for anchoring locally in said mass.
According to another characteristic of the invention, given the position occupied by each reinforcing element, the length of its portion working in traction is preferably equal to or greater than the distance in the corresponding horizontal plane between the back face of the facing and the corresponding local portion of the potential slip plane.
According to another characteristic of the invention, at most two longitudinal bars of the mesh constituting the anchoring portion extend as far as the facing to form the portion that works in traction.
According to another characteristic of the invention, only two longitudinal bars of the mesh extend as far as the facing to form the portion working in traction and they are spaced apart by spacers.
According to a characteristic of the invention that is different from the preceding characteristic, the portion working in traction comprises two longitudinal strips that are not interconnected.
According to a characteristic relating more particularly to the retaining wall, the attachment parts of the reinforcing element are placed in the immediate vicinity of a rib so as to reduce the bending forces applied to the transverse connection bar.
According to another characteristic of the invention, each prefabricated facing element is of a height substantially equal to the height of the level or stage of filling for which it constitutes the facing.
According to yet another characteristic of the invention, the rib of the prefabricated facing element is of a height that is less than that of said element.
According to yet another characteristic of the invention, each reinforcing element comprises two longitudinal bars forming the portion that works in traction, these two longitudinal bars being spaced apart in such a manner that their attachment parts can be placed on either side of a rib of a facing element while being situated close to said rib.